Recently, curing of ink by radiation, and in particular ultraviolet (UV) curing, has become popular. UV curable ink can be cured after printing by application of UV light. Typically, UV curable inks include monomers that form polymers by free radical polymerization. The growing end of each polymer chain is a radical that reacts with additional monomers, transferring the radical to the end of the chain as each monomer is added. A photo initiator is used to form the first radicals to begin the polymerization process. The photo initiator is capable of absorbing UV light to generate radicals to react with the monomers.
Two types of photo initiators can be used in UV curable compositions. Type I photo initiators are unimolecular photo initiators that undergo a hemolytic bond cleavage upon absorption of UV light, forming radicals. Type-II photo initiators are bimolecular photo initiators. These are used as a system of a photo initiator with a synergist, which can together form radicals upon exposure to UV light. Some type-II photo initiators react by hydrogen abstraction from the synergist to the photo initiator.